1. Field of the Invention
The present invention relates to an optical-fiber preform produced according to a modified chemical-vapor deposition process, and more particularly to an optical-fiber preform having barrier layers to prevent hydroxyl radicals (OH radicals).
2. Description of the Related Art
An optical-fiber preform is a raw material used for fabricating an optical fiber. The optical-fiber preform includes a core layer with a high refractive index, a clad layer surrounding an outer surface of the core layer, and a quartz tube in the form of cylindrical tube serving as a substrate for the optical-fiber preform. The core layer has a refractive index higher than that of the clad layer. Thus, an optical signal incident on the core layer causes a total reflection at an interface between the clad layer and the core layer, so that the transmission of the optical signal is achieved in the core layer.
In order to produce the optical-fiber preform, a method known as a modified chemical-vapor deposition is widely used, in which chemical reactants resulting from a thermal reaction are deposited on the inner surface of a quartz tube to form an optical preform. During the modified chemical-vapor deposition, an inside of the quartz reaction tube is maintained at a high temperature by heating the quartz reaction tube and then a raw material gas is introduced into the quartz reaction tube. As such, a chemical reactant is produced by heating the raw material gas onto the inner surface of the quartz reaction tube. Accordingly, the modified chemical-vapor deposition is performed by depositing the raw material gas onto the inner surface of the quartz reaction tube by heating the quartz reaction tube, collapsing the quartz reaction tube by heating it above a softening temperature, and closing the quartz reaction tube. This method is widely used for producing an optical-fiber preform of a hill type because it is easy to control the distribution of the refractive index.
In the above mentioned method, however, an oxygen/hydrogen burner heating a quartz tube, which contains hydroxyl radicals (—OH), generates hydroxyl radicals during the process of manufacturing the optical-fiber preform. The hydroxyl radicals permeate inside the optical-fiber preform due to a concentration gradient between the quartz reaction tube, the clad layer, and the core layer. The hydroxyl radicals that permeate inside the optical-fiber preform cause changes in the refractive index of the clad layer or the core layer. As a result, errors disturbing the optical signal occur. Moreover, the hydroxyl radicals that permeate the core layer tend to form a non-bridging oxygen, thereby degrading the uniformity in the construction of the core layer. Accordingly, the presence of the hydroxyl radicals leads to many disorders and disturbance in the optical signal.
FIG. 1 shows a construction of an optical-fiber preform having a barrier layer for the hydroxyl radicals and a profile of a refractive index of the optical-fiber preform. As shown in FIG. 1, the optical-fiber preform 110 includes a quartz tube 113, a clad layer 112, a core layer 111 located at the center of the optical-fiber preform 110, and a barrier layer 114. Note that hydroxyl radicals are deposited between the quartz reaction tube 113 and the core layer 111.
The quartz reaction tube 113 serves as a substrate when producing the optical-fiber to preform 110 and is produced using a sol-gel method. The clad layer 112 surrounds an outer surface of the core layer 111 and has a distribution of the refractive index that is lower than that of the core layer 111. Accordingly, an optical signal incident on the core layer 111 causes a total reflection within the core layer 111 for transmission. The barrier layer 114 for the hydroxyl radicals includes the composition of SiO2 or SiO2 +GeO2 and prevents the hydroxyl radicals from permeating the core layer 111 and the clad layer 112.
Referring to the profile of the refractive index of the optical-fiber preform 110, the core layer 111 has the highest refractive index, and the clad layer 112 has the lowest refractive index. The refractive-index distribution of the barrier layer 114 is lower than that of the core layer 111 but higher than that of the clad layer 112. Meanwhile, the refractive index distribution of the quartz reaction tube 113 is higher than that of the barrier layer 114 but lower than that of the core layer 111. As shown in FIG. 1, the barrier layer 114 and the clad layer 112 have negative refractive-index distributions (ΔN−) with reference to the refractive index of the quartz reaction tube 113. In contrast, the core layer 111 has a positive refractive-index distribution (ΔN+) greater than the refractive index of the quartz reaction tube 114.
U.S. Pat. No. 6,280,850, which is entitled with “Optical fiber preform having OH barrier and Manufacturing method thereof” and issued to Sungkuk Oh, discloses a construction of an optical-fiber perform, which includes barriers to hydroxyl radicals disposed between a quartz tube and a clad layer and between the clad layer and the core layer in order to prevent the hydroxyl radical from permeating the core layer. In particular, material such as SiO2, GeO2, etc., is employed as the barrier to the hydroxyl radicals. In addition, the barrier deposited between the clad layer and the core layer is doped with F, so that the refractive index thereof is regulated to equal that of the clad layer.
However, the optical-fiber preform having the conventional barrier to the hydroxyl radicals as stated above induces a self-collapsing problem due to excessive heating, which reduces the inner diameter of a quartz reaction tube and the diameter of the core at the center of the quartz reaction tube. This prevents heat from being smoothly transferred while the optical-fiber preform is turning to glass. Furthermore, the reduction of the inner diameter of the quartz reaction tube leads to the reduction of the thickness of the core layer capable of being deposited, which in turn reduces the length of the optical fiber capable of being drawn from a unit of preform.
Therefore, there is a need for an optical-fiber perform with an improved barrier layer capable of overcoming the above-mentioned drawbacks.